Such alloys as defined by particular compositions are described in ONERA's French Pat. No. 2 555 204 (equivalent to U.S. Ser. No. 363,285, filed Mar. 29, 1982). These alloys compare favorably with the best superalloys known previously with respect to high temperature creep resistance, and they also have significantly lower density.
The above French patent mentions, in particular, the highest performance superalloys then in production, known under the reference PWA 1422 (DS 200+Hf), having a density of 8.55 g/cm.sup.3 (grams per cubic centimeter). This is an alloy having column-shaped grains obtained by directed solidification and having high strength against forces acting in a direction parallel to the boundaries between the grains.
Said patent also refers to an alloy resulting from recent work on the development of novel compositions suitable for making monocrystalline blades by directed solidification. This alloy known under the reference PWA 1480 (or alloy 454) has a density of 8.7 g/cm.sup.3.
The resistance of these prior alloys to creep when hot is obtained in conventional manner by massive additions of refractory elements such Ta, W, Mo, or Re. Thus, alloy 454 contains 12% Ta and 4% W, and alloy DS 200+Hf contains 12% W.
These refractory elements play an important part in reducing creep rate, thereby providing a proportional increase in operating lifetime.
Even at high temperatures, these elements have very low diffusion rates, thereby slowing down the coalescence rate of the hardening or gamma prime phase Ni.sub.3 (Al, Ti, . . . ) on which hot creep resistance depends.
However, these refractory elements are very heavy and even if they do increase hot creep resistance, they suffer from the drawback of simultaneously increasing the density of the alloy.
The alloy density could be reduced by adding large quantities of light elements, and in particular of aluminum, however this would give rise to primary precipitation of the gamma prime phase, and the alloy would not have the required creep characteristics.
In order to reconcile the contradictory requirements relating to density and to creep resistance, work done by the Applicant concerning superalloys for turbomachine blades has led to the optimization of two parameters S.sub.1 and S.sub.2 respectively related to the concentration of refractory elements and to the concentration of elements which participate in the formation of the hardening gamma prime phase, namely: EQU S.sub.1 =0.5 W+Ta+Mo
where the chemical symbols represent weight percentages of the corresponding elements, and EQU S.sub.2 =Al+Ti +Ta+Nb+V
where the chemical symbols represent percentages in terms of numbers of atoms of the elements.
The above-specified patent proposes alloy compositions in which S.sub.1 lies in the range 4% to 9% by weight and S.sub.2 lies between 14.9% and 20.6% of the atoms.
The quantity of vanadium in the sum S.sub.2 serves to enlarge the heat treatment window, i.e. the temperature range between the end of the gamma prime phase being put into solution and the starting melting point of the alloy, and this can be advantageous when heat treatment is performed in an industrial context.
C, B, and Zr are not incorporated in these alloys in order to avoid reducing the starting melting temperature of the alloy and thus to allow the part to be raised during heat treatment to a sufficiently high temperature for the gamma prime phase to be put back into solution together with substantially all of the gamma/gamma prime eutectic, with the combination then being precipitated during cooling in the form of fine gamma prime particles.
Patent No. 2 555 204 proposes the following composition where the percentages are by weight:
______________________________________ Co 5 to 7% Cr 5 to 10% Mo 0.5 to 2.5% W 0 to 3.5% Al 6 to 7.5% Ti 1.5 to 2.25% Nb 0 to 0.5% Ta 2 to 4% V 0.3 to 0.6% Ni balance to 100% without voluntary addition of B, C, ______________________________________ Zr.
After making the alloy into a monocrystalline blade, the blade is subjected to heat treatment to put the gamma prime phase into solution. This treatment consists in raising the part to a temperature lying in the range 1290.degree. C. to 1325.degree. C., depending on its composition, for a period of time lying between thirty minutes and four hours.
The part is then cooled in air. Heat treatment, as defined in French Pat. No. 2 503 188 filed Apr. 3, 1981 (equivalent to U.S. continuation Ser. No. 878,401 filed June 20, 1986), is then applied to precipitate the gamma prime phase.
This precipitation takes place at a temperature of more than 1000.degree. C.
A regular distribution of gamma prime particles having an average size of 0.5 microns is thus obtained.
The precipitates are aligned along crystallographic directions of the&lt;100&gt;type.
The relative density of such alloys is about 8.2.
It has been observed that alloys in accordance with patent No. 2 555 204 which contain at least 0.3% vanadium do not stand up well to cyclic oxidation.
Although, in practice, turbine blade alloys are nearly always covered with a protective layer against corrosion and oxidation, it is important that the naked material is also capable of standing up well to environmental conditions in order to avoid accelerated degradation of the alloy whenever the coating is damaged.
The invention is based on the surprising observation that some vanadium-free alloys not only have considerably higher creep resistance characteristics than the alloys described in examples 1 and 2 of the above-mentioned patent (alloys 1 and 2 whose compositions are reproduced in table 1 below), but also have much better resistance to cyclic oxidation at 1100.degree. C. than said alloys 1 and 2. Even more surprisingly, the heat treatment window of these vanadium-free alloys is about 20.degree. C., i.e. practically the same as for alloys 1 and 2. It is thought that this result is due mainly to the very low concentration of boron in alloys in accordance with the invention (not more than 10 ppm), which means that there are hardly any drawbacks in doing without vanadium.
The invention relates to alloy compositions in which the parameter S.sub.1 as defined above lies between 7.8% and 8.5% by weight and the parameter S.sub.2 lies between 15.6% and 16.88% of the atoms.